Camera tube having photoconductive lead monoxide layer on silicon carbide signal plate

ABSTRACT

In order to ensure satisfactory durability of signal windows for use in electron tubes comprising a transparent substrate covered by a radiation-sensitive layer, the contact surface of the substrate with said layer is rendered electrically conductive with the aid of conductive silicon carbide.

ilnite States Patent 1 91 Knippenberg et al. 5]March 20, 1973 CAMERA TUBE HAVING [56] References Cited PHOTOCONDUCTIVE LEAD MONOXIDE LAYER N SILICON UNHED STATES PATENTS CARBIDE SIGNAL PLATE 2,698,912 1/1955 Teves et a1 ..313/65 R 3,444,412 /1969 De Haan et a1 ..313/65 A 1 Inventors: Wllhelmus Franclsws pp 2,544,755 3/1951 Johnson et a] .513/010. 7 Gerrit Verspui, both of Emmasingel, 3,310,700 3/ 1967 Dresner et a1. ..313/65 A Eindhoven, Netherlands 3,505,551 4/1970 Bynum ..313/65 A 3,577,285 5/1971 Rutz ..317/235 AP Asslgneel g- -k 2 1 Corporation, New 3,600,645 8/1971 Berman ..317/235 or 221 Filed: Nov. 18, 1970 57 g 'fggqg t S6881 t0rneyran r1 an [21] Appl. No.: 90,493

[57] ABSTRACT Foreign Application Priority Data In order to ensure Satisfactory durability of signal windows for use in electron tubes comprising a trans- Dec. 19, 1969 Netherlands ..6919053 parent Substrate covered y a radiatiomsensitive layer, the contact surface of the substrate with said layer is [52] US. Cl. .3l3/65 A, 313/311 rendered electrically conductive with the aid of com [5 l] lnt. 3 ductive i i carbide [58] Field of Search ..313/ A, 65 AB, 65 R 2 Claims, 1 Drawing Figure XXXXXXXXXXXXXX PATH-1TH] "AR 2 01973 I N VE NTORS KNl PPENBERG WILHELMUS F. GERRIT VERSPUI /76 f/VT CAMERA TUBE HAVING PHOTOCONDUCTIVE LEAD MONOXIDE LAYER ON SILICON CARBIDE SIGNAL PLATE The invention relates to a signal window for use in electron tubes, particularly cathode-ray tubes. In the first place camera tubes of the Vidicon type and oscillograph tubes are concerned here; the signal window allows in the first case the incoming light and in the second case the emitted light to pass.

The windows of such tubes are formed by a transparent substrate provided with a layer of a radiationsensitive substance for image recording. The light-pervious substrate is electrically conductive at least at the surface provided with the radiationsensitive substance so that it may be used for conducting away the electric charge and as an electrode.

The term radiation-sensitive substance" is to be understood a substance capable of producing reversible alterations of electric properties such as the conductivity or the dielectric properties such as the conductivity or the dielectric constant or of the light emission under the action of electro-magnetic or corpuscular radiation. The term therefore covers both photo-conductive and luminescent substances.

Transparent, electrically conductive surfaces may, as is known, be obtained by the vapor-deposition of a metal, particularly precious metals on the substrate. It is, however, a disadvantage that thin metal layers do not exhibit a uniform light transmission throughout the spectrum. For example, thin gold layers have a blue to green color.

Materially better in this respect are the known, colorless, electrically conductive layers of tin oxide and indium oxide. For many purposes these oxide layers also exhibit adequate chemical resistance. However, it has been found that in contact with the metal compounds frequently used for radiation-sensitive layers for example, photoconductive compounds such as lead oxide and antimony trisulphide and luminescent compounds such as willemite the chemical inertia of the conductive layers of, for example, tin oxide and indium oxide, much to be desired.

The object of the invention is to provide a signal window for use in an electron tube, which does not exhibit this disadvantage.

According to the invention this result is achieved with the aid of silicon carbide which has proved to be useful in connection with its particular chemical inertia for various purposes in technology. It is known that silicon carbide can be rendered adequately electrically conductive by the incorporation of donors and/or acceptors while its transparency is maintained.

The invention relates to a signal window for use in an electron tube, particularly a camera tube of the Vidicon type, said window being formed by a transparent substrate covered on one side by a layer of a radiation-sensitive substance for image recording, said substrate being electrically conductive at least on the surface covered by the radiation-sensitive layer and is characterized in that the substrate consists at least on said side of compact silicon carbide with a content of donors and/or acceptors.

The signal windows according to the invention may comprise a substrate completely consisting of compact silicon carbide and being electrically conductive at least on one surface.

It is known to obtain bodies of compact silicon carbide by sublimation, by gaseous phase reactions, for example, pyrolysis of a metalchlorosilane in the presence of hydrogen and by converting bodies of graphite, silicon or quartz wholly or superficially by a reaction with silicon or carbon or compounds thereof respectively into silicon carbide.

It is preferred to use substrates of quartz covered with a compact layer of silicon carbide, since quartz bodies of the required dimensions are available and are readily converted superficially into silicon carbide.

The electric conductivity of at least the surface of silicon carbide may, as stated above, be achieved by the incorporation of donors and/or acceptors. It is preferred to use additions which at the most produce a slight discoloration of the silicon carbide so that the light transmission throughout the spectrum can be obtained practically without hindrance. The incorporation of the additions may be performed during the formation or the deposition of the silicon carbide via the gaseous phase or by diffusion.

The windows according to the invention may be sealed with the aid of intermediate glass to further parts for completing the electron tubes. For this purpose borosilicate glass may be used when the window substrate consists completely of silicon carbide. For sealing substrates of a quartz body converted superficially into silicon carbide the conventional intermediate kinds of glass known for sealing quartz to glass are employed.

The electrically conductive silicon carbide of the windows may be locally provided with electric contacts by melting down metals such as alloys of gold and tantalum, nickel and molybdenum, and platinum and tin.

The invention will now be set out with reference to the accompanying drawing.

The FIGURE of the drawing shows, by way of example, schematically in a longitudinal sectional view a camera tube of the Vidicon type provided with a signal window embodying the invention.

The camera tube shown comprises an exhausted, elongated, cylindrical bulb l of glass, closed at one end by the substrate 2 of the signal window and at the other end by the glass foot 3. This foot, a central portion of which is formed by a sealed exhaust tube 4, is provided with metal pins 5, which are electrically connected in the bulb to various parts of an electrode system designated as a whole by 6.

The electrode system 6 comprises a thermionic cathode 8 to be heated by a filament wire 7, a Wehnelt cylinder 9 and a perforated anode 10, which is electrically connected to a cylindrical electrode 1 l. The electrode 11 is provided on the side of the window with a gaze electrode 12.

The signal window comprises a transparent substrate 2 of silicon carbide or quartz, provided on the side facing the electrode 1 1 with a transparent signal electrode 13 formed by a compact layer of electrically conductive silicon carbide. The substrate body 2 with the electrode 13 is sealed, as stated above, in the cylindrical bulb 1 if necessary with the aid of one or more intermediate kinds of glass. The signal electrode 13 is provided with a current conductor 14 taken to the outside.

To the signal electrode 13 is applied from the vapor phase in an atmosphere of oxygen and water vapor a photo-conductive layer 15 of a thickness of about 15 to 20 um oflead monoxide (PbO).

The layer 15, which forms the target plate of the tube, can be scanned on the face opposite the electrode by an electron beam 16 emanating from the cathode 8. With the aid of the conventional deflection and focusing coils, designated in common by 17, surrounding the tube, the electron beam 16 is focused on the target plate and caused to perform a scanning movement.

By means of an optical system represented schematically by a single lens 18 an image to be converted into electrical signals is focused in operation on the target plate 15. The electrical signals are obtained, as usual, during the scan of the target plate by the electron beam 16, across a signal resistor 19 included in the current conductor 14, through which signal resistor the signal electrode 13 receives a bias voltage of 20 to 60 V, which is positive to the cathode 8.

Hereinafter a few examples are given of substrates for signal windows embodying the invention and of the method of manufacturing the same.

EXAMPLE 1 Pure silicon carbide, obtained by pyrolysis of a methyl-chlorosilane, was sintered in a graphite mould in argon having less than 1 ppm of nitrogen at 2,200C for 1 hour into a cylindrical body of an outer diameter of 70 mms an inner diameter of 40 mms and a height of 100 mms.

The cylinder was subsequently heated for 5 hours at 2,500C in a graphite crucible in helium containing less than 0.1 ppm of nitrogen, care being taken that the temperature gradient along the wall of the crucible was slight both in an axial and a radial sense.

Large, clear, plate-shaped crystals of pure silicon carbide grew on the inner wall of the cylinder.

Crystals of 10 X 10 X 1 mm were provided on one surface by the diffusion of aluminum with a thin, transparent, conductive layer of a resistivity of 0.5 Ohm.cm.

Wafers are made from the crystals and provided with contacts by melting down strips of gold and 5 percent of tantalum on the electrically conductive surface.

The wafers are sealed in tubes of borosilicate glass, the contacts of the gold alloy being taken through the tube wall.

EXAMPLE 2 In a graphite crucible having a plate-shaped lid of a diameter of 70 mms, also of graphite, pure silicon carbide powder was heated at 2,600C in an atmosphere of argon with 0.01 percent of nitrogen. At a temperature of 2,400C of the lid a dense, quite light green layer of silicon carbide sublimated thereon to a thickness of 4 mms within 5 hours. After the graphite had been burnt away in an oxygen atmosphere, wafers were formed by grinding and sawing, which wafers may be used as conductive substrates for signal windows. The resistivity of the substrates rendered conductive by nitrogen doping was 1 Ohm.cm.

The wafers were provided with contacts by melting down strips of an alloy of nickel with l0 percent of molybdenum and sealed in a tube of borosilicate glass, the contacts being taken to the outside.

EXAMPLE 3 A graphite plate of a diameter of 40 mms was heated at l,5 50C in a flow of hydrogen of 10 liters a minute, 1 percent by volume of methylchlorosilane.

containing Within 1 hour a layer of yellowish transparent, pure silicon carbide of a thickness of 1 mm was deposited on the plate. Owing to the nitrogen in the gas stream, was was partly absorbed in the silicon carbide, the latter was electrically conductive. The resistivity was 200 Ohm.cm.

After the graphite plate had been burnt away in an oxygen atmosphere at 1,000C and after grinding and polishing a disc of a diameter of 40 mms and a thickness of 0.7 mm was obtained, which may be employed as a window substrate and may be processed further in the manner described in Examples 1 and 2.

EXAMPLE 4 A quartz disc of a diameter of 20 mms and a thickness of 3 mms. was covered on one side with a thin layer of a carbon suspension.

After drying the assembly was heated at l,l00C in a hydrogen atmosphere for 8 hours. The quartz was thus superficially converted into silicon carbide. The disc was subsequently heated again for 2 hours in oxygen with 1 percent volume of nitrogen. The residual carbon was thus burnt away and at the same time an electrically conductive surface was obtained due to the presence of the nitrogen in the oxygen atmosphere.

In the manner described in the preceding Examples this window was provided with contacts. For sealing in glass the conventional intermediate kinds of glass are employed.

The window substrates are provided with radiationsensitive layers. If the signal windows are manufactured for use in camera tubes of the Vidicon type, a photoconductive layer, particularly a vapor-deposited lead monoxide layer is provided. For the manufacture of oscillograph tubes conventional layers of luminescent substances for example, willemite are applied.

What is claimed is:

1. An electron discharge tube comprising an evacuated envelope having a light transmissible quartz wall portion, a photoconductive layer of lead monoxide having a thickness of 15 to 20 pm covering one side of said wall portion for recording and reproducing an image, an electron beam source within said envelope, means to scan said photoconductive layer with said electron beam, and a transparent compact electricallyconductive layer of silicon carbide between said photoconductive layer and said wall portion.

2. An electron discharge tube as claimed in claim 1 in which the transparent layer of electrically conductive silicon carbide is provided with a metal contact for making an electrical connection thereto. 

2. An electron discharge tube as claimed in claim 1 in which the transparent layer of electrically conductive silicon carbide is provided with a metal contact for making an electrical connection thereto. 